1. Field of the Invention
The present invention relates to a phoretic display incorporating a liquid crystal suspension medium having a preferred configuration therein and to a method of achieving the preferred configuration of the liquid crystal suspension medium within the phoretic display. Such a phoretic display may be used in a wide range of electronic consumer goods, for example household electrical and electronic items, mobile telephones, personal computers, electronic personal organizers, e-book etc.
2. Discussion of Prior Art
Phoretic displays typically comprise a suspension layer containing finely divided phoretic particles dispersed within a liquid suspension medium. The spatial distribution of the phoretic particles within the suspension layer may be altered by applying a magnetic or electric field to the device. The particles migrate within the device under the influence of the applied field.
A phoretic display device exhibiting non-linear optical behavior may be achieved using a phoretic cell incorporating a liquid crystal suspension medium with a phoretic particle suspended therein. The above mentioned phoretic cell and phoretic display are described in further detail in UK patent application GB 0218776.3
In particular an electrophoretic display may be fabricated using an electrophoretic system in which particles of an electrophoretic material are suspended in a liquid crystal suspension medium and where the geometry of the system is arranged to promote defects within the liquid crystal material. A defect may be thought of as a local break in the orientational symmetry of the liquid crystal material, for example a local break in the orientational order of the liquid crystal director. Such defects are sometimes referred to as disclinations in the liquid crystal material.
The liquid crystal alignment is highly distorted in the region immediately surrounding each defect. Hence, there is a large amount of elastic energy associated with each defect. For the purposes of this patent specification this elastic energy shall also be referred to as the defect energy.
The defects within the liquid crystal material are configured to attract electrophoretic particles; each defect giving rise to an associated region of influence within the liquid crystal material. Once within a particular region of influence, an electrophoretic particle is attracted towards the defect associated with that region of influence. If unobstructed, the electrophoretic particle will move within the region of influence to adopt a preferred position where the elastic energy due to the defect is minimised.
The defects create a threshold level in the device. An applied electric field must exceed this threshold, for a sufficient time period, in order to remove a particle from the region of influence associated with a defect. A useful display device may be achieved by selectively positioning the defects in the liquid crystal material at the surface(s) of the display. An image may be formed on the display by varying the spatial distribution of the electrophoretic particles at the defects on the display surface(s). A bistable or multi-stable display may be formed by arranging defects on opposing surfaces of the display.
One technique for selectively positioning the defects at the surface(s) of the display consists of disposing a relief structure at the display surface(s). The profile of the relief structure typically comprises an array of indentations, with each indentation creating a defect site and a preferred position for an electrophoretic particle within the liquid crystal material.
The relief structure surface is adapted to provide a preferred orientation of the liquid crystal material within the display. Typically, the preferred orientation of the liquid crystal material comprises an essentially non-homeotropic alignment of the liquid crystal director at the relief structure surface, i.e. a tangential component of the liquid crystal director is encouraged giving a tilted or planar alignment. The preferred orientation may be achieved by applying a suitable surface alignment treatment to the relief structure.
Notwithstanding the efficacy of the foregoing, circumstances may arise where a simple surface alignment treatment is insufficient to provide the preferred orientation of the liquid crystal material at the relief structure surface. For example, depending upon the particular geometry of the relief structure surface, undesirable parasitic effects may conspire against the creation and correct positioning of defects sites within the phoretic cell.
In particular, problems may be encountered in configurations where the preferred alignment of the liquid crystal material gives rise to strong localised distortions of the liquid crystal director. The distortions within the liquid crystal director create a high degree of elastic or defect energy within the material which can lead to instability of the configuration and to the potential failure of the display.
It is an object of the present invention to mitigate at least some of the disadvantages of the above phoretic display. It is a further object of the present invention to provide a method for achieving the preferred configuration of the liquid crystal suspension medium within the phoretic display.